The present invention relates to a cell flow control unit and method for use in asynchronous transfer mode (ATM) switching or transmission networks.
As means to use efficiently existing communication networks for transmitting, exchanging or transferring (the three functions may hereinafter be generically referred to as transmitting) multi-media information including speech, data and moving picture information, the synchronous transfer mode (STM) and the packet mode are commonly used. The STM uses time division multiplexing, which periodically allocates time slots irrespective of the presence or absence of information to be sent, and allows a single process of communication to occupy exclusively a channel of a certain transmission rate. Therefore, while the transmission rate is fixed over a whole transmission network, the utilization rate of each channel is generally low. These problems are serious in high speed wide band communication network, particularly with communication in a broad band integrated services digital network (ISDN), where the types of information to be transmitted increase with the transmission rate and the frequency bandwidth of the communication network and accordingly a greater diversity of transmission rates comes to be required. On the other hand, in the packet mode, the information to be sent is divided into blocks, each of which is transferred in a packet augmented with a header which indicates routing information. Since a packet is sent as information arises, the transmission rate can be chosen as desired. Transmission in the packet mode, however, requires a complex protocol, and software processing for its execution hinders transmission rate improvement. This problem poses a serious constraint when, for instance, multi-media information involving a large quantity of information including moving picture information is to be transmitted on a real time basis.
In contrast to the STM and the packet mode, which inevitably entail these problems, the ATM, which is an improved packet mode having a unified format in which the packets have a fixed length, makes it possible to simplify said software processing, to increase the transmission rate, and, accordingly, to provide a high speed wide band communication network such as a high speed local area network (LAN) or a wide band ISDN. In view of this advantage, research and development attempts for commercial application of the ATM are being energetically undertaken. A cell for use in ATM transmission usually has a header length of 5 bytes (40 bits) and an information region length of 48 bytes (384 bits) in accordance with the CCITT Recommendations I.150 and I.361. The ATM, using cell multiplexing on the basis of header information, permits a substantial change in the transmission rate of multi-media information through a high speed transmission path (link) of 156 Mbps in transmission rate. Thus, there is used a self-routing switch which selects the cell transmission path (virtual path or virtual channel) on the basis of header information, and the ATM places this switch control under distributed processing. Since the processor for network control has no direct part in this switch control, high speed transmission of multi-media information is made possible.
Architecture of a high speed wide band communication network for high speed transmission of multi-media information in the ATM indispensably requires a cell flow control function to limit the flow rate of cells with an ATM layer having a high throughput, because uneven distribution of traffic would adversely affect the utilization efficiency of the whole transmission network. A cell flow control technique for this purpose is described in U.S. Pat. No. 4,956,839. This cell flow control circuit counts the number of cells transferred within a predetermined period of time on the basis of header information and, when the count surpasses a predetermined maximum, either discards the excess cells or transfers them to the self-routing switch, with the header being converted to indicate that they are an excess. The switch, receiving the transfer of the excess cells, discards them according to the level of traffic at the switch. This way of flow control, which determines whether given cells are an excess or not according to the maximum as the only parameter, is susceptible to the influence of traffic variations. As it entails the processing of header information, which requires complex circuit elements, for excess cell discarding but also is dependent on the self-routing switch downstream for part of the excess cell discarding, the self-routing switch which has to be simple in circuit configuration in order to ensure high speed is complicated by the addition of a buffer memory and otherwise.